The present study describes the synthesis of mesoporous silica nanoparticles using a modified sol-gel method. Various proportions of acetonitrile-water mixtures were utilized so as to optimize the reaction mixture for facile synthesis of mesoporous silica nanoparticles with controlled particle size for the very first time. After carefully adjusting the water and acetonitrile contents i.e. to 1:1 v/v ratio, a more uniform and small sized nanoparticles were achieved. The resultant particles were 140 nm in size having pore size of approximately 5.9 nm and were safe to be used in the cellular system, as confirmed by the in vitro cytotoxicity studies.